Increasing topology without affecting shape keys
Dissolve X, Vertex Connect Path J, Knife Topology Tool K

With the Basis key active and all other keys set to 0.0, these operators allow me to thoroughly retopologize a character model (~18,000 vertices) and maintain shape key deformation and vertex weights long after rigging and adding almost one hundred shape keys. They use the current surface detail of the mesh to retain shape key/vertex group information, as opposed to destructive operators like Delete which have tendencies force a global recalculation of surface detail.